The trade unionists had been pushing for eight-hour days, not five-day weeks. The thing Ford did largely unprompted was switch from six days to five days. Even your own link seems to confuse the two, but is focused on the eight-hour day part of the original false image.
> Ford’s initiative was not widely copied overnight. In 1916, the federal government passed an act to require an eight-hour day and overtime pay for railroad workers, but most workers still didn’t have those protections, and working hours remained a hotly contested issue. "Demands for the five-day week began to proliferate in 1919, a year in which 4 million American workers went out on strike,"